Stray light estimates for the TPF formation flying interferometer

摘要

A formation flying interferometer suffers a unique stray light problem: it is the only space astronomical instrument which looks at starlight in the presence of a bright object (the sunlit collector spacecraft) which is intentionally placed in the field of view. The combiner instrument is a collection of astronomical telescopes looking at stellar targets relayed by the collectors; but other parts of the collector spacecraft can scatter or emit substantial fluxes. Screening the glare while collecting the starlight is the challenge. The TPF formation-flying interferometer is especially vulnerable because the planets it seeks are so faint. We present a preliminary estimate of stray light from various sources appearing in the interferometer science detector. We assumed a pinhole field stop would be employed, but that two mirrors preceding that pinhole in the combiner optical system would scatter off-axis light into the pinhole. Sources include scattering of direct sunlight and thermal emission, both from the multi-layer thermal shades which permit passive cooling of the instruments to about 40K. We find that the estimated stray light fluxes are of order 10~4 times the planet flux. We conclude that the optics at the combiner entrance must be blocked from any direct view of the thermal shades.